Their #1 artist? Are you saying that you still think Mariah is their '#1 artist or #1 priority'? It's obviously Rihanna b/c she's been the one churning out hit singles consistently since her 2nd album. I would think that if you combine digital single sales and album sales, Rihanna has probably outside Mariah since TEOM. So Rihanna IS their #1 priority not Mariah. 
How many top 10s does Rihanna have? I think she might be up there with Beyonce.